双端队列可以在队列的两端进行插入和删除操作，既可在队尾进行插入/删除，又可在队头进行插入/删除。现有4个不同的元素顺序输入到双端队列，那么可以得到_____种不同的排列。 double-ended queue can insert and delete operations on both ends of the queue. That it can insert / delete at its